Relecting radio waves... parabola the best?

[loic2003]

I'm looking to transmit the signal from a wireless video camera as far as possible. It's got a little aerial about 5 inches long and transmits at 1.2GHz. Since the receiver isn't going to move, I was thinking of reflecting all the signal being produced in the same direction towards the receiver, thus increasing the signal strength and the range.

My current plan is to build a parabolic shape out of some thin conductive metal that goes along the whole length of the aeriel. The aerial would be placed along the focus of the parabola.

similarly, I could build the equivalent device for the receiver unit.

Is this the best way to do what I aim to do? What materials reflect radio waves better? will any conducting metal do fine? What would be the best size of parabola to use? would I want the focus point to be a certain distance from the reflector?

This apparatus will hopefully be mounted to a balloon to transmit aeriel photographs of an area, so weight is a big issue. I was thinking of creating a wire frame and using baco-foil as the reflective material...

I've posted this question here as I know that wireless networking uses a similar frequency and there have been lots of experiemnts to figure out the maximum range possible, whihc could be useful to me! Suggestions and any other help would be much appreciated!

 

[Peter]

If it's on a balloon, you'll definitely NOT want too focussed a beam, else you'll be missing your receiver more often than not.

 

[Kc2iLq]

With a parabolic reflector, you are focusing the signal to achieve gain over a halfwave dipole. In order to achieve the gain, you are taking all of the stray radiowaves and pointing them in one direction. Therefore, you would have to point the dish directly at the receiving antenna. However, you can not stabilize it by any means if it is on a balloon. Your best bet would be to use a high gain vertical like a Comet GP-21 so you have a good, clean signal. The transmitter will be high in the air, so there will be minimal interference. You shouldn't have a problem if your signal is over three watts.

-Kc2iLq

 

[Lynx516]

You are going to have a lisence though probably if your signal is over 3W

 

[Kc2iLq]

Originally posted by: Lynx516
You are going to have a lisence though probably if your signal is over 3W

Heh, I never really thought about that. I got an Amauter Radio license so I can do all the experimenting I want in the GHz range (for the most part).

-Kc2iLq

 

[Lynx516]

I think the Max transmtion power without a licence is 500mW (for unlicenced bands) at least this is what it is in the UK

 

[Hayabusa Rider]

You really want to look at amateur radio plans for parabolic antennas. You cannot just stick a parabolic reflector behind an antenna. You must provide a proper feed. You can google on feeds and get some idea of what you are going to need.

 

[loic2003]

See, this is just a really simple piece of kit that I'm using. It's output power is a whopping.... 50mW, but it's good for 100 -> 150m range (line of sight) here's a link to a similar model:

http://www.gadgets4you.com/ProductDetails.asp?PID=173

We've got it working pretty well, but now want to strap it to several helium balloons and get some really neat aerial footage of the area that I live in... I was thinking of strapping a tinfoil parabola-like shape above the horizintally mounted aerial, and a larger, simialr shape to the receiver aerial... the shape would be a little wider than a parabola so that the signal is not too concentrated....

I was just thinking of increasing the range a bit by concentrating the signal that is sent upwards and in unwanted directions down towards the receiver on the ground... this way maybe I could squeeze an extra 50m out of it without the signal dying...

I'll post up results when I go through with it!

cheers

 

[DrPizza]

The overlooked solution:

Get more helium balloons. Attach the receiver and a laptop to the 2nd set of balloons and keep them within the proper range of the camera. It's gonna take a LOT of balloons though!

 

[brentkiosk]

Ioic,

I've been wondering about your project since lthis thread started. Will look forward to your results.

Concerning the reflector. A curved reflector is a llot like the reflector for a flashlight or car headlight. It does direct the majority of the power into a beam. You can think of your system as having a flashlight on the ground and one on the balloons. If you can keep them aimed at each other, you will get some gain. If they miss, then you will probably end up behind. I think you don't want a very narrow beam from the balloon. But, you will know a lot more when you try it. Let us know what happens.

 

[Witling]

This raises a question for me. The focus of a parabola is a point, not a five inch axis. The antenna was described as being five inches long. Does an antenna (or can it be considered to) radiate from a point?

 

[brentkiosk]

Witling, That is an issue. A 5 inch antenna is not a a point, and that affects the radiation (and receive) pattern. Also, you are dealing with waves, not straight-line rays. Both of these facts make the pattern you get different than a point source at the focus of a parabola would indicate. But, the point/parabola model isn't a terrible approximation to start with. Using a parabolic reflector can focus the beam making it stronger in a selected direction.

There's a lot to radio antenna design. You can always get "gain" by reducing radiation in some directions and making it go where you want it to. There are whole books on different ways to do this. Using a dish reflector is pretty common at the 1.2 Ghz frequency mentioned. Until the dish size gets to be 20 or 50 times the wavelength (the wavelength is about 10 inches here) a sphere will work about as well as a parabola. As the gain is increased in one direction, the spread of the beam becomes smaller, and lining things up becomes more critical.

 

[JediJeb]

I am probably way off here but since the antenna is 5in long could you use a parabolic trough reflector similar to those used behind the quartz heater tubes in electric space heaters to direct the IR in one general direction. While not focused to a point it would bounce all the energy in the same general direction ( downward from the baloons) which would in effect cause close to a doubling of the signal aimed towards the ground. Like I said I am probably way off the mark here, but I would like to know if that would work.

 

[JPSJPS]

Parabolic/corner reflectors (narrow beam):
http://www.freeantennas.com/

Flat Plate reflectors (wide beam):
http://www.freeantennas.com/2400/

 

[Abzstrak]

a parabolic reflector is going to a bitch in the wind. I take it that the balloons will be teathered right? so just make a small yagi antenna that points straight down and another on the ground pointing up at the ballon. drag won't be nearly as bad and if designed right you should be able to pick up 12dB or maybe even 15dB gain. You could even cut the directors and reflectors from a single piece of aluminum, just make the driven element something else like a piece of copper (easy to solder to) obviously insulated from the piece of aluminum.

You know, if it is teathered just tie the end of the stupid yagi to the string and it will always aim perfectly at the teather base (obviously discounting for the hyperbolic curve of the string). that way even in high winds it will always aim correctly (of course use a similar design on the ground)

This seems much easier to me and should get sufficient gain for your project